The disclosure relates to a laser system using Raman scattering.
With miniaturization and high integration of a semiconductor integrated circuit, an improvement in resolution has been demanded for a semiconductor exposure apparatus. Hereinafter, the semiconductor exposure apparatus is simply referred to as an “exposure apparatus”. Shortening in a wavelength of light to be outputted from an exposure light source has been in progress accordingly. A gas laser device is used in place of an existing mercury lamp for the exposure light source. Currently, a KrF excimer laser device and an ArF excimer laser device may be used as gas laser devices for exposure. The KrF excimer laser device may output ultraviolet light of a wavelength of 248 nm, and an ArF excimer laser device may output ultraviolet light of a wavelength of 193 nm.
As a next generation exposure technology, liquid immersion exposure has been studied in which a clearance between an exposure lens on exposure apparatus side and a wafer is filled with a liquid and a refractive index is changed to shorten an apparent wavelength of light derived from the exposure light source. When the liquid immersion exposure is performed using the ArF excimer laser device as the exposure light source, ultraviolet light of a wavelength of 134 nm in water may be applied to the wafer. This technology is referred to as ArF liquid immersion exposure. The ArF liquid immersion exposure may also be referred to as ArF liquid immersion lithography.
Since a spectral line width in free oscillation of each of the KrF excimer laser device and the ArF excimer laser device is wide, e.g., in a range from about 350 pm to about 400 pm, color aberration and its consequential decrease in resolution occur when projection lenses of the KrF excimer laser device and the ArF excimer laser device are used. It is therefore necessary to narrow a spectral line width of a laser beam to be outputted from the gas laser device to an extent in which the color aberration is negligible. The spectral line width is also referred to as a spectral width. Accordingly, a line narrowing module including a line narrowing device is provided in a laser resonator of the gas laser device to achieve narrowing of the spectral width. Non-limiting examples of the line narrowing device may include an etalon and a grating. The laser device that allows for narrowing of the spectral width is referred to as a line narrowing laser device.
Further, development of a unit that generates vacuum ultraviolet (VUV) light of a wavelength equal to or lower than an wavelength of an ArF excimer laser has been in progress. Since few laser media produce VUV light, wavelength conversion is necessary to produce VUV light. Typical wavelength conversion methods may include wavelength conversion using a nonlinear crystal and wavelength conversion using Raman scattering. For example, reference is made to U.S. Pat. No. 5,771,117 and Andrew J. Merriam, S. J. Sharpe, H. Xia, D. Manuszak, G. Y. Yin, and S. E. Harris, “Efficient gas-phase generation of coherent vacuum ultraviolet radiation”, OPTICS LETTERS, Vol. 24, No. 9 (1999) 625-627.